As an easy method for measuring a glucose level in blood, a disposable biosensor is used (See JP-B2-8-10208, for example). An example of such biosensor is shown in FIGS. 11 and 12 of the present application. In the illustrated biosensor 9, a responsive current which is necessary for the computation of a blood glucose level is measured by utilizing a working electrode 90 and a counter electrode 9. The biosensor 9 includes a substrate 92, and a cover 94 stacked on the substrate via a spacer 93 formed with a slit 93a. These components 92-94 define a capillary 95 on the substrate 92. The capillary 95 is utilized for moving blood by capillary action and retaining the blood. The capillary 95 communicates with the outside through an introduction port 96 for introducing blood and an air vent 97 for discharging air from the capillary 95 when the blood moves in the capillary 95.
On the substrate 92 are provided an insulating layer 98 and a reagent portion 99. The insulating layer 98 covers the working electrode 90 and the counter electrode 91 while exposing opposite ends 90a, 90b of the working electrode 90 and opposite ends 91a, 91b of the counter electrode 91. The reagent portion 99 covers the ends 90a, 91a of the working electrode 90 and the counter electrode 91 and is in a solid state containing oxidoreductase and an electron mediator.
To measure a blood glucose level, with a biosensor 9 mounted to a concentration measuring apparatus (not shown), blood BL is introduced into the capillary 95 through the introduction port 96, as shown in FIG. 13. In the capillary 95, the movement of the blood BL stops at the edge 97a of the air vent 97, and the reagent portion 99 is dissolved by the blood BL, whereby a liquid phase reaction system is established. A voltage can be applied to the liquid phase reaction system by the power source of the concentration measuring apparatus through the working electrode 90 and the counter electrode 91. The responsive current upon the voltage application can be measured at the blood glucose level measuring apparatus (not shown) by utilizing the working electrode 90 and the counter electrode 91. The responsive current is obtained as the reflection of the amount of electrons transferred between the electron mediator and the end 90a of the working electrode 90 in the liquid phase reaction system. Thus, the responsive current relates with the amount of the electron mediator which exists around the working electrode 90 and which is capable of transferring electrons between the end 90a of the working electrode 90.
However, when the concentration measurement is performed by using the biosensor 9, the measurement result is sometimes higher than the actual concentration. To find out the cause, the inventors of the present invention measured the change of oxidation current with time by using some samples. As a result, it is found that, in some cases, as is in the time course of oxidation current shown in FIG. 14, the oxidation current, which should decreases monotonically in normal circumstances, suddenly increases instantaneously, as circled in the figure. When such a phenomenon happens to occur at the time point for measuring the oxidation current for computing the blood glucose level, the computation result becomes higher than the actual blood glucose level.
The inventors of the present invention checked a plurality of samples in which the above-described phenomenon was seen. As a result, it was found that, as a feature common to these samples, blood BL had reached beyond the edge 97a of the air vent 97 on the surface of the substrate 92, as shown in FIG. 15. On the other hand, in the samples in which the sudden increase of the oxidation current did not occur, blood BL was stopped at the edge 97a of the air vent 97 (See FIG. 13).
Conceivably, from the above difference, the sudden increase of oxidation current is caused by the remove of the blood BL, i.e., the phenomenon that the blood BL once stopped at the edge 97a of the air vent 97 moves beyond the edge 97a of the air vent 97.
Specifically, when voltage is applied to the liquid phase reaction system including the blood BL, electrons are transferred between the electron mediator and the end 90a of the working electrode 90. Therefore, in the state in which the movement of the blood BL is suspended, the proportion of reductant is low at the surface of the end 90a of the working electrode 90, so that the oxidation current decreases. In this state, when the blood BL moves, reductant moves from the introduction port 96 side to the surface of the end 90a of the working electrode 90, so that the proportion of the reductant at the surface of the end 90a temporarily increases. As a result, the amount of electrons transferred between the reductant and the surface of the end 90a of the working electrode 90 suddenly increases, so that the oxidation current does not monotonically decrease but increases temporarily.